Host Name: Hostname is used to connect to a mongod instance. This hostname can be a hostname, an FQDN, an IPv4 address, or an IPv6 address.
Replica Set Name: Replica Set is a set of servers with the copies of data. Usually, it is a set of 3 mongod servers with 1 Primary and 2 Secondary servers. So, replica set name is the name given for whole replica set. More information is given here in docs-replication. And there is M103 course which covers the topic in detail.
Kanika